<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>

       .box{
           width: 100px;
           height: 100px;
            background-color: greenyellow;
            float: left;
       }

        /*浮动清除 ： left right both*/
       .clearfix{
          clear: both;
       }
  </style>
</head>
<body>
      <div class="box">
          box
      </div>
<div class="clearfix">
  hello
</div>

</body>
</html>